1. Field of the Invention
The invention relates to a misfire determination system for an internal combustion engine, a vehicle, a system for estimating the rigidity of a torsion element, a misfire determination method for an internal combustion engine, and a method of estimating the rigidity of a torsion element. More specifically, the invention relates to an internal-combustion-engine misfire determination system for determining the occurrence of a misfire in a multi-cylinder internal combustion engine of which the output shaft is connected, through a torsion element, to a downstream shaft downstream of the torsion element, a vehicle including such a misfire determination system, a system for estimating the rigidity of a torsion element that is interposed between the output shaft of a multi-cylinder internal combustion engine and a downstream shaft, a misfire determination method for determining the occurrence of a misfire in an internal combustion engine, and a method of estimating the rigidity of a torsion element.
2. Description of the Related Art
A misfire determination system for an internal combustion engine has been proposed that, in a vehicle in which vibration control is performed using an electric motor so as to cancel fluctuation in torque (rotational fluctuation) of a crankshaft of an engine, calculates the amount of adjustment of torque that is output from the electric motor for vibration control, and detects misfiring in the engine based on the amount of adjustment of torque made by the electric motor (see Japanese Patent Application Publication No. 2001-65402 (JP-A-2001-65402), for example).
In a misfire determination system installed in a vehicle in which a damper (torsion element) is interposed between the crankshaft of the engine and a downstream shaft, fluctuation in torque of the crankshaft due to explosive combustion in the engine induces resonance of the torsion element and the downstream components including the torsion element. The resonance causes rotational fluctuation of the crankshaft, and as a result, even when it is tried to detect the occurrence of a misfire in one of the cylinders of the engine based on the rotational fluctuation of the crankshaft, the occurrence of a misfire cannot be accurately detected. The rotational fluctuation of the crankshaft that is caused by resonance is affected by the rotational speed of the crankshaft and the rotational speed of the downstream shaft, and in addition, affected by the rigidity (spring constant) of the torsion element. The manufacturing error and the chronological change are large with respect to the spring constant of the torsion element, and it is therefore desirable to accurately estimate the spring constant when the rotational fluctuation of the crankshaft caused by resonance is analyzed.